MSE 321

MECHANICAL BEHAVIOUR OF MATERIALS

Yahya K. Tür

Things to Know

Sources : A comprehensive list will be given at the end of the lecture

In Turkish: Malzemelerin yapısı ve mekanik davranışları, Kayalı ve

Çimenoğlu, İTÜ Kimya Metalurji Fakültesi

Grading : Midterm (40 %) + Final (60%)

Tentative Midterm Date: November 3, 2017

One A4 sheet hand written notes is permitted in the exams.

Second sheet and photocopied sheets are to be collected.

Office hours: Wednesday 13:00-14:00 Room : MLZ 208, Phone: 605 2640Email : yktur@gtu.edu.tr

Mechanical Behaviour of Materials

Part I Mechanical fundamentals

• Stress and strain relationships of elastic behavior

• Plastic deformation

Part II Metallurgical fundamentals

• Plastic deformation of single crystals

• Dislocation theory

• Strengthening mechanisms

• Fracture and Fatigue

Part III

•Mechanical behavior of ceramics

•Mechanical behavior of polymers.

•Mechanical behavior of composites

Objective: Macro and micro mechanical behaviour of materials under the

influence of external forces

Common States of Stress• Simple tension: cable

A o = cross sectional

area (when unloaded)

FF

o

s =F

Ass

Note: t = M r/Jo here.

o

t =F s

AM

M A o

2R

F sA c

• Torsion (a form of shear): drive shaft Ski lift (photo courtesy

P.M. Anderson)

OTHER COMMON STRESS STATES (1)

(photo courtesy P.M. Anderson)Canyon Bridge, Los Alamos, NM

A o

Balanced Rock, Arches National Park o

s =F

A

• Simple compression:

Note: compressivestructure member(s < 0 here).(photo courtesy P.M. Anderson)

• Bi-axial tension: • Hydrostatic compression:

Pressurized tank

s < 0h

(photo courtesyP.M. Anderson)

(photo courtesyP.M. Anderson)

OTHER COMMON STRESS STATES (2)

Fish under water

s z > 0

sq

> 0

Why failure in materials

• Seven of the Liberty Ships built during

the world war II has broken completely

in two as a result of brittle fractures.

• Over 1000 of approximately 5000

merchant ships built during World War

II had developed cracks of considerable

size by 1946.

Failure of Liberty Ships during services inWorld War II.

Why failure in materials

• The bridge building industry did not

pay particular attention to the

possibility of brittle failure until the

failure of Point Pleasant bridge in 1967.

• The bridge collapsed without

warning, costing 46 lives.

Collapse of Point Pleasant suspensionbridge, West Virginia, on December 15, 1967.

Why failure in materials

• The aircraft was used for interisland

transportation for 19 years before failed.

• Failure has been attributed to

multiple-site-damage.

Failed fuselage of the Aloha 737 aircraft in 1988.

Material property assessments• Hardness :

Micro/Macro hardness tests

• Strength, Ductility (elongation, reduction of area):

Tension tests, Bending tests

• Creep (elevated temperature strength):

Creep tests

• Torsion:

Torsion tests

• Toughness (resistance to failure) :

Impact tests; Fracture toughness tests

• Fatigue:

S-N fatigue tests;Fatigue crack growth tests

Hardness tests

• Hardness is a property which is a measure of a resistance topermanent or plastic deformation.

increasing hardness

most plastics

brasses Al alloys

easy to machine steels file hard

cutting tools

nitrided steels diamond

• Large hardness means:--resistance to plastic deformation or cracking in compression.--better wear properties.

Stress-Strain Testing (Tensile Test)

Adapted from Fig. 6.3, Callister 7e. (Fig. 6.3 is taken from H.W. Hayden, W.G. Moffatt, and J. Wulff, The Structure and Properties of Materials, Vol. III, Mechanical Behavior, p. 2, John Wiley and Sons, New York, 1965.)

specimenextensometer

• Typical tensile specimen

Adapted from Fig. 6.2,Callister 7e.

gauge length

• Typical tensile test machine

• Stress at which noticeable plastic deformation hasoccurred.

when ep = 0.002

Yield Strength, sy

sy = yield strength

Note: for 50 mm sample

Adapted from Fig. 6.10 (a),Callister 7e.

tensile stress, s

engineering strain, e

sy

e p = 0.002

e = 0.002 = z/z

z = 0.1 mm

Tensile Strength, TS

• Metals: occurs when noticeable necking starts.• Polymers: occurs when polymer backbone chains are

aligned and about to break.

Adapted from Fig. 6.11, Callister 7e.

sy

strain

Typical response of a metal

F = fracture or

ultimate

strength

Neck – acts as stress concentrator

engi

ne

erin

g

TSst

ress

engineering strain

• Maximum stress on engineering stress-strain curve.

Impact Tets

• Measure toughness of materials in terms of energy absorption.

• Specimen is impacted by a hammer and theenergy absorbed during fracture is measuredin Joule.

Fracture mechanics

• Resistance of materials to crack

propagation (to failure).

• Crack propagation can be

predicted before failure.

• Material will fail when the

stress intensity factor K

reaches the critical value KC.

Fatigue tests

Material is subjected to a repetitive or

fluctuating stress (cyclic loading) and will fail at

a stress level much lower than that causes

failure in statistic loading.

Stresses in fatigue loading

Parameters:• Fracture life (fatigue strength)• Fatigue crack growth resistance• Paris exponent (m)• Fatigue threshold (Kth)

To Improve Material Properties

Main references

• Dieter, G.E., Mechanical metallurgy, 1988, SI metric edition, McGraw-Hill, ISBN 0-07-100406-8.

• Hibbeler, R.C. Mechanics of materials, 2005, SI second edition, Person Prentice Hall, ISBN 0-13-186-638-9.

•Hertzberg, R. W., Deformation and Fracture Mechanics of Engineering Materials, 1995, Wiley; 4 edition, ISBN-13: 978-0471012146

•Callister, W. D. Jr. , Materials Science and Engineering: An Introduction, 2007, John Wiley & Sons, Inc. , ISBN-13: 978-0-471-73696-7

•Meyers, M. A. and Chawla, K. K., Mechanical Behavior of Materials, 2009, Cambridge University Press, ISBN-13 978-0-521-86675-0

•Udomphol, T., Mechanical Metallurgy lecture notes, http://www.sut.ac.th/engineering/Metal/courses/mechmet.html

• Rösler, J. ,· Harders , H., · Baeker M., Mechanical Behaviour of EngineeringMaterials 2007, Springer, ISBN 978-3-540-73446-8